NFC is a very short range wireless communication system. It can transfer small amounts of data wireless between two devices that are a few centimeters from each other.
NFC is an extension of Radio Frequency IDentification (RFID) technology and allows two-way communication between two devices while basic RFID only allows one-way communication.
NFC is often directed to short information exchange between an active NFC device, e.g. a mobile phone and an accessory NFC device. The accessory NFC device may be a unit or tag that harvests its power from the active NFC device. Such information can be just identification of said unit or tag or it can be collecting information from the unit of tag. Identification of a unit can also be the first step in automated establishment of other communication means such as Bluetooth communication or Wi-Fi communication. Other uses of NFC is automated use of applications of many kinds such as enabling of applications in mobile phones, replacing credit cards, unlocking door locks etc. To perform such tasks, small chunks of information are transmitted in a short time.
NFC typically uses a frequency of 13.56 MHz for communicating. The corresponding wavelength is about 22 meters. The antennas of the active NFC device and the accessory NFC device are in fact not real antennas but rather electrical coils. Still we will call them antennas. This results in a very short communication range—in the area of 0 to about 4 centimeters. This also implies that some small amount of power can be transferred between the devices, normally from the active NFC device to the accessory NFC device. Then the accessory NFC device may harvest this power to perform its intended tasks without using any other power source.
Typically e.g. a mobile phone may set up an intermittent continuous NFC field in a state that we may call sniffer mode. In sniffer mode, the average current needed for this from the mobile phone is rather small, typically in the area of one mA. The sniffer mode is typically a repetitive short duration pulse train that is repeated at intervals of a few hundred milliseconds. An electromagnetic field then surrounds the mobile phone. When an accessory NFC device approaches the mobile phone acting as an active NFC device, the active NFC device detects that the impedance of its antenna changes and will start a procedure to investigate and connect the active and the accessory NFC devices.
When the two NFC devices are connected, data exchange mode is enabled. In data exchange mode, a short chunk of information is transferred one or both ways, the exchange mode is terminated, and sniffer mode is reinstated.
NFC-technology is based on standards including ISO 18092, ISO 21481, ECMA (340, 352 and 356) and ETSI TS 102 190. Furthermore, such NFC-technology is also compatible with contactless infrastructure for smartcards based on a standard ISO 14443 A, including Philips' MIFARE-technology and Sony's FeliCa-card.
One example in the art that presents harvesting of power from a second NFC device to a first NFC device is the Patent publication US 20140170976 A1 of Broadcom Corporation. This application uses the existing NFC technology as specified in the current standards.
Existing NFC enabled mobile telephones use often about 1 milliampere to the task of maintaining an NFC sniffer modus and about 200 milliampere when in data exchange mode. Because NFC usually is in data exchange mode for a relatively short period of time of less than one minute, this is not a considerable power consumption. On the other hand, if the data exchange mode is extended to more than one minute, ten minutes or even more than one hour, this is exhaustive power drawn from e.g. a mobile telephone running on internal batteries.
Existing NFC enabled mobile telephones use often about 1 milliampere to the task of Sniffer modus is used by mobile telephones more or less continuous in order to react to an approaching second NFC device. Mobile telephones are designed to be operable a long time between charging of its batteries and therefore the added power consumption in NFC sniffer modus is of particular importance. Much effort, including filing of a number of patent applications, has been done to minimize necessary power in this modus. Connected mode is normally just used just for a short time while transmitting a transaction act, e.g. transferring money or verifying an airplane ticket or allowing passing through a gate. Therefore an increased power consumption during such short times are normally not a problem for a mobile phone battery.
When data is transferred between two NFC devices, a transmitter enters the data on an NFC carrier wave by amplitude modulation.
The patent application US 2010130127 A1 of Sony describes a system prepared to turn off output of an electromagnetic wave for a given period of time. The turn off time is based on attribute information indicating communication ability. This turning off is due to a target of reducing power consumption e.g. in a cell phone. In connected mode, on the other hand, this application does not reveal reducing or turning off output of the magnetic wave in connected mode.
The patent application US 2009196209 A1 presents an implementation of a power savings mode in which a transmitting device sleeps after transmitting a first synchronization packet. Optionally also after transmitting a second synchronization packet.
An important aim of the present invention is not only to save power in a first NFC device, but to improve power harvesting, i.e. to make sure that a second active NFC device harvests sufficient power from the first NFC device to operate, even without the second NFC device necessarily comprising power supplied from conventional secondary or primary batteries. This invention does the power harvesting more reliably while at the same time consuming less power than present solutions do.